Brushless motor and disk drive apparatus

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a brushless motor and a disk drive apparatus.

2. Description of the Related Art

A hard disk apparatus is equipped with a brushless motor for rotating a disk. A conventional brushless motor is disclosed in, e.g., SG135981. A permanent-magnet synchronous motor of SG185981 includes a permanent magnet mounted to at least one of a top yoke and a bottom yoke and an armature arranged within an air gap between the permanent magnet and the other yoke (see claim <b>1</b>).

Referring to FIG. 4 of SG135981, a yoke is attached to a rotor shell. A permanent magnet is attached to the yoke. In this structure, the yoke is interposed between the rotor shell and the permanent magnet. For this reason, it becomes difficult to reduce the axial thickness of the motor. In an effort to make the motor thinner, there is provided, e.g., a structure in which a rotor shell is made of a magnetic material and in which a permanent magnet is accommodated in a recess portion formed on the lower surface of the rotor shell.

However, if the permanent magnet is accommodated in the recess portion arranged on the lower surface of the rotor shell, magnetic fluxes are apt to flow from the permanent magnet toward the lower surface of the rotor shell positioned radially inward or radially outward of the permanent magnet. In this case, the amount of magnetic fluxes flowing from the permanent magnet toward an armature is decreased.

SUMMARY OF THE INVENTION

A brushless motor according to one illustrative embodiment of the subject application includes a stationary unit and a rotary unit rotatably supported with respect to the stationary unit. The stationary unit includes an armature and a base member arranged to support the armature. The armature has a flat shape and extends in a direction orthogonal to a center axis extending in an up-down direction. The rotary unit includes a magnet, a hub made of a magnetic material and a rotor yoke made of a magnetic material. The magnet is positioned above the armature. The hub is arranged to hold the magnet. The rotor yoke is provided with a portion positioned below the armature. The armature includes a plurality of coil patterns arranged along a circumferential direction. The hub includes an inner lower surface, an outer lower surface and a recess portion. The inner lower surface is positioned radially inward of the magnet. The outer lower surface is positioned radially outward of the magnet. The recess portion is arranged between the inner lower surface and the outer lower surface and is depressed upward. At least a portion of the magnet is accommodated within the recess portion. At least one of the inner lower surface and the outer lower surface is positioned above a lower surface of the magnet.

According to one illustrative embodiment of the subject application, at least a portion of the magnet is accommodated within the recess portion. This helps reduce the axial dimension of the brushless motor. Moreover, at least one of the inner lower surface and the outer lower surface is positioned above the lower surface of the magnet. This makes it difficult for the magnetic fluxes to flow from the magnet toward the inner lower surface or the outer lower surface. As a result, the magnetic fluxes can efficiently flow from the magnet toward the armature.

A brushless motor according to another illustrative embodiment of the subject application includes a stationary unit and a rotary unit rotatably supported with respect to the stationary unit. The stationary unit includes an armature and a base member arranged to support the armature. The armature has a flat shape and extends in a direction orthogonal to a center axis extending in an up-down direction. The rotary unit includes a magnet, a hub made of a magnetic material and a rotor yoke made of a magnetic material. The magnet is positioned above the armature. The hub is arranged to hold the magnet. The rotor yoke is provided with a portion positioned below the armature. The armature includes a plurality of coil patterns arranged along a circumferential direction. The hub includes a lower surface and a recess portion depressed upward from the lower surface. At least a portion of the magnet is accommodated within the recess portion. The recess portion includes an inner side surface and an outer side surface. The inner side surface is positioned radially inward of the magnet. The outer side surface is positioned radially outward of the magnet. A radial gap exists in at least one of a space between a radial inner end surface of the magnet and the inner side surface and a space between a radial outer end surface of the magnet and the outer side surface.

According to another illustrative embodiment of the subject application, at least a portion of the magnet is accommodated within the recess portion. This helps restrain the axial dimension of the brushless motor from becoming larger. Moreover, a radial gap exists in at least one of a space between a radial inner end surface of the magnet and the inner side surface and a space between a radial outer end surface of the magnet and the outer side surface. This makes it difficult for the magnetic fluxes to flow from the magnet toward at least one of the inner side surface and the outer side surface. As a result, the magnetic fluxes can efficiently flow from the magnet toward the armature.

The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a vertical section view showing a brushless motor according to a first preferred embodiment.

<figref idref="DRAWINGS">FIG. 2</figref> is a vertical section view showing a disk drive apparatus according to a second preferred embodiment.

<figref idref="DRAWINGS">FIG. 3</figref> is a vertical section view showing a brushless motor according to the second preferred embodiment.

<figref idref="DRAWINGS">FIG. 4</figref> is a cutaway perspective view of the brushless motor according to the second preferred embodiment.

<figref idref="DRAWINGS">FIG. 5</figref> is a perspective view showing an armature according to the second preferred embodiment.

<figref idref="DRAWINGS">FIG. 6</figref> is a partial vertical section view showing the brushless motor according to the second preferred embodiment.

<figref idref="DRAWINGS">FIG. 7</figref> is a partial vertical section view showing the brushless motor according to the second preferred embodiment.

<figref idref="DRAWINGS">FIG. 8</figref> is an exploded perspective view showing a hub and a magnet according to the second preferred embodiment.

<figref idref="DRAWINGS">FIG. 9</figref> is a perspective view showing the hub and the magnet according to the second preferred embodiment.

<figref idref="DRAWINGS">FIG. 10</figref> is a partial vertical section view of a brushless motor according to one modified example.

<figref idref="DRAWINGS">FIG. 11</figref> is a partial vertical section view of a brushless motor according to another modified example.

<figref idref="DRAWINGS">FIG. 12</figref> is a perspective view showing a hub and a magnet according to a further modified example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, illustrative embodiments of the present invention will now be described with reference to the accompanying drawings which form a part hereof. In the following description, the direction parallel to the center axis of a brushless motor will be referred to as “axial”. The direction orthogonal to the center axis of the brushless motor will be referred to as “radial”. The direction extending along an arc about the center axis of the brushless motor will be referred to as “circumferential”. In the following description, the shape and positional relationship of the respective portions will be described under the assumption that the axial direction extends in an up-down direction and that the side of a magnet with respect to an armature is an upper side. However, such definition of the up-down direction is not intended to limit the in-use direction of the brushless motor and the disk drive apparatus according to the present invention.

In the following description, the term “parallel” is intended to include a substantially parallel direction. Likewise, the term “orthogonal” is intended to include a substantially orthogonal direction.

<figref idref="DRAWINGS">FIG. 1</figref> is a vertical section view showing a brushless motor <b>11</b>A according to a first preferred embodiment. As shown in <figref idref="DRAWINGS">FIG. 1</figref>, the brushless motor <b>11</b>A preferably includes a stationary unit <b>2</b>A and a rotary unit <b>3</b>A. The rotary unit <b>3</b>A is rotatably supported with respect to the stationary unit <b>2</b>A.

The stationary unit <b>2</b>A preferably includes a base member <b>21</b>A and an armature <b>22</b>A. The base member <b>21</b>A is arranged to support the armature <b>22</b>A. The armature <b>22</b>A extends in a flat shape in the direction orthogonal to a center axis <b>9</b>A. The armature <b>22</b>A includes a plurality of coil patterns arranged along a circumferential direction. The rotary unit <b>3</b>A preferably includes a hub <b>32</b>A made of a magnetic material, a magnet <b>34</b>A and a rotor yoke <b>35</b>A made of a magnetic material. The magnet <b>34</b>A is positioned above the armature <b>22</b>A. The magnet <b>34</b>A is held by the hub <b>32</b>A. The rotor yoke <b>35</b>A has a portion positioned below the armature <b>22</b>A.

As shown in <figref idref="DRAWINGS">FIG. 1</figref>, the lower surface of the hub <b>32</b>A preferably includes an inner lower surface <b>81</b>A, an outer lower surface <b>82</b>A and a recess portion <b>83</b>A. The inner lower surface <b>81</b>A is positioned radially inward of the magnet <b>34</b>A. The outer lower surface <b>82</b>A is positioned radially outward of the magnet <b>34</b>A. The recess portion <b>83</b>A is arranged between the inner lower surface <b>81</b>A and the outer lower surface <b>82</b>A and is depressed upward. At least a portion of the magnet <b>34</b>A is accommodated within the recess portion <b>83</b>A. Thus, the axial dimension of the brushless motor <b>11</b>A is restrained from becoming larger.

In the present embodiment, the inner lower surface <b>81</b>A and the outer lower surface <b>82</b>A are positioned above the lower surface of the magnet <b>34</b>A. This makes it difficult for magnetic fluxes to flow from the magnet <b>34</b>A toward the inner lower surface <b>81</b>A and the outer lower surface <b>82</b>A. As a result, it becomes easier for the magnetic fluxes to efficiently flow from the magnet <b>34</b>A toward the armature <b>22</b>A. Only one of the inner lower surface and the outer lower surface may be arranged above the lower surface of the magnet.

The recess portion <b>83</b>A preferably includes an inner side surface <b>831</b>A and an outer side surface <b>832</b>A. The inner side surface <b>831</b>A is positioned radially inward of the magnet <b>34</b>A. The outer side surface <b>832</b>A is positioned radially outward of the magnet <b>34</b>A. In the present embodiment, as shown in <figref idref="DRAWINGS">FIG. 1</figref>, a radial gap <b>92</b>A exists between the radial outer end surface of the magnet <b>34</b>A and the outer side surface <b>832</b>A. This makes it difficult for the magnetic fluxes to flow from the magnet <b>34</b>A toward the outer side surface <b>832</b>A. As a result, the magnetic fluxes can efficiently flow from the magnet <b>34</b>A toward the armature <b>22</b>A. A radial gap may exist between the radial inner end surface of the magnet and the inner side surface. In this case, it becomes difficult for the magnetic fluxes to flow from the magnet toward the inner side surface.

<figref idref="DRAWINGS">FIG. 2</figref> is a vertical section view showing a disk drive apparatus <b>1</b> according to a second preferred embodiment. The disk drive apparatus <b>1</b> is an apparatus for performing information reading and writing tasks with respect to a magnetic disk <b>12</b> while rotating the magnetic disk <b>12</b>. As shown in <figref idref="DRAWINGS">FIG. 2</figref>, the disk drive apparatus <b>1</b> preferably includes a brushless motor <b>11</b>, a magnetic disk <b>12</b>, an access unit <b>13</b> and a cover <b>14</b>.

The brushless motor <b>11</b> supports the magnetic disk <b>12</b> and rotates the magnetic disk <b>12</b> about the center axis <b>9</b>. The brushless motor <b>11</b> preferably includes a base member <b>21</b> extending radially at the lower side of the magnetic disk <b>12</b>. The rotary unit <b>3</b> of the brushless motor <b>11</b>, the magnetic disk <b>12</b> and the access unit <b>13</b> are accommodated within a housing. The housing is made up of the base member <b>21</b> and the cover <b>14</b>. The access unit <b>13</b> displaces a head <b>131</b> along the recording surface of the magnetic disk <b>12</b>. This enables the access unit <b>13</b> to perform information reading and writing tasks with respect to the magnetic disk <b>12</b>.

The disk drive apparatus <b>1</b> may include two or more magnetic disks <b>12</b>. The access unit <b>13</b> may perform only one of the information reading and writing tasks with respect to the magnetic disk <b>12</b>.

Next, description will be made on the configuration of the brushless motor <b>11</b>. <figref idref="DRAWINGS">FIG. 3</figref> is a vertical section view of the brushless motor <b>11</b>. <figref idref="DRAWINGS">FIG. 4</figref> is a cutaway perspective view of the brushless motor <b>11</b>. As shown in <figref idref="DRAWINGS">FIGS. 3 and 4</figref>, the brushless motor <b>11</b> preferably includes a stationary unit <b>2</b> and a rotary unit <b>3</b>. The stationary unit <b>2</b> is kept stopped with respect to the base member <b>21</b> and the cover <b>14</b>. The rotary unit <b>3</b> is rotatably supported with respect to the stationary unit <b>2</b>.

The stationary unit <b>2</b> of the present embodiment preferably includes a base member <b>21</b>, an armature <b>22</b>, a sleeve <b>23</b> and a cap <b>24</b>.

The base member <b>21</b> supports the armature <b>22</b> and the sleeve <b>23</b>. The base member <b>21</b> is formed by casting metal, e.g., aluminum. As shown in <figref idref="DRAWINGS">FIG. 3</figref>, the base member <b>21</b> preferably includes a bottom plate portion <b>51</b>, a base protrusion portion <b>52</b> and an armature holding portion <b>54</b>.

The bottom plate portion <b>51</b> lies below the armature <b>22</b> and extends in the direction orthogonal to the center axis <b>9</b>. The base protrusion portion <b>52</b> extends upward from the inner periphery of the bottom plate portion <b>51</b> in a substantially cylindrical shape. The base protrusion portion <b>52</b> is positioned radially inward of a hub protrusion portion <b>62</b> to be described later. A base through-hole <b>53</b> axially extending through the base member <b>21</b> is arranged radially inward of the base protrusion portion <b>52</b>.

The armature holding portion <b>54</b> lies radially outward of a rotor yoke <b>35</b> to be described later and protrudes upward from the bottom plate portion <b>51</b>. The armature holding portion <b>54</b> preferably includes a wall portion <b>541</b> and an annular surface <b>542</b>. The wall portion <b>541</b> extends in a substantially cylindrical shape in a coaxial relationship with the center axis <b>9</b>. The annular surface <b>542</b> extends radially inward from the lower end of the wall portion <b>541</b>. The armature <b>22</b> is arranged radially inward of the wall portion <b>541</b> and is supported on the annular surface <b>542</b>.

The armature <b>22</b> is a flat circuit board extending in the direction orthogonal to the center axis <b>9</b>. The armature <b>22</b> is arranged radially outward of a bearing mechanism <b>4</b>, below a magnet <b>34</b> to be described later and above a flange portion <b>72</b> of a rotor yoke <b>35</b> to be described later. <figref idref="DRAWINGS">FIG. 5</figref> is a perspective view showing the armature <b>22</b>. As shown in <figref idref="DRAWINGS">FIG. 5</figref>, the armature <b>22</b> preferably includes a ring-shaped plate portion <b>221</b> and a protruding plate portion <b>222</b>. The ring-shaped plate portion <b>221</b> lies below the hub <b>32</b> and the magnet <b>34</b> to be described later and extends in an annular shape. The protruding plate portion <b>222</b> has a flat shape and protrudes radially outward from a portion of the radial outer edge of the ring-shaped plate portion <b>221</b>.

An electronic circuit is arranged on the upper or lower surface of the ring-shaped plate portion <b>221</b>. The electronic circuit preferably includes a plurality of coil patterns <b>223</b> arranged along the circumferential direction. For example, each of the coil patterns <b>223</b> is arranged in a spiral pattern about a coil axis extending in an axial direction. A power feeding portion <b>224</b> is arranged on the lower surface of the protruding plate portion <b>222</b>. When an electric current is to be fed to the disk drive apparatus <b>1</b>, a lead wire extending from an external power source is connected to the power feeding portion <b>224</b>. Then, a drive current is supplied from the lead wire to the coil patterns <b>223</b> via the power feeding portion <b>224</b>.

As shown in <figref idref="DRAWINGS">FIGS. 3 and 4</figref>, the sleeve <b>23</b> extends axially in a substantially cylindrical shape around a shaft <b>31</b> to be described later. The lower portion of the sleeve <b>23</b> is inserted into the base through-hole <b>53</b> and is fixed to the inner circumferential surface of the base protrusion portion <b>52</b>. The inner circumferential surface of the sleeve <b>23</b> is radially opposed to the outer circumferential surface of the shaft <b>31</b>. The lower opening of the sleeve <b>23</b> is closed by the cap <b>24</b>.

The rotary unit <b>3</b> of the present embodiment preferably includes a shaft <b>31</b>, a hub <b>32</b>, a ring-shaped member <b>33</b>, a magnet <b>34</b> and a rotor yoke <b>35</b>. The shaft <b>31</b> is arranged radially inward of the sleeve <b>23</b> and the hub <b>32</b> to extend in the axial direction. The shaft <b>31</b> is made of metal, e.g., stainless steel. The shaft <b>31</b> is supported on the sleeve <b>23</b> and the cap <b>24</b> through a lubricant <b>41</b>. The shaft <b>31</b> rotates about the center axis <b>9</b>. Th upper end portion of the shaft <b>31</b> protrudes upward beyond the upper surface of the sleeve <b>23</b>. The shaft <b>31</b> and the hub <b>32</b> may be a continuously extending member.

The hub <b>32</b> preferably includes a top plate portion <b>61</b>, a hub protrusion portion <b>62</b> and a disk support portion <b>63</b>. The hub <b>32</b> is made of a magnetic material. The top plate portion <b>61</b> lies above the armature <b>22</b> and extends radially and circumferentially. The radial inner edge portion of the top plate portion <b>61</b> is fixed to the upper end portion of the shaft <b>31</b>. The magnet <b>34</b> is held on the lower surface of the top plate portion <b>61</b>. The hub protrusion portion <b>62</b> extends downward from the top plate portion <b>61</b> in a substantially cylindrical shape.

The disk support portion <b>63</b> is arranged radially outward of the top plate portion <b>61</b> to support the magnetic disk <b>12</b>. The disk support portion <b>63</b> preferably includes a substantially cylindrical first support surface <b>631</b> and a second support surface <b>632</b> extending radially outward from the lower end of the first support surface <b>631</b>. At least a portion of the inner circumferential portion of the magnetic disk <b>12</b> makes contact with the first support surface <b>631</b>. Thus, the magnetic disk <b>12</b> is radially located in position. At least a portion of the lower surface of the magnetic disk <b>12</b> makes contact with the second support surface <b>632</b>. Accordingly, the magnetic disk <b>12</b> is axially located in position.

The ring-shaped member <b>33</b> is a circular ring-shaped member positioned radially inward of the hub protrusion portion <b>62</b>. The upper surface of the ring-shaped member <b>33</b> makes contact with the lower surface of the top plate portion <b>61</b>. The radial outer surface of the ring-shaped member <b>33</b> is fixed to the hub protrusion portion <b>62</b>, e.g., by press fit or by an adhesive agent.

A lubricant <b>41</b> exists between the combination of the sleeve <b>23</b> and the cap <b>24</b> and the combination of the shaft <b>31</b>, the hub <b>32</b> and the ring-shaped member <b>33</b>. The liquid level of the lubricant <b>41</b> is positioned between the outer circumferential surface of the sleeve <b>23</b> and the inner circumferential surface of the ring-shaped member <b>33</b>. The shaft <b>31</b>, the hub <b>32</b> and the ring-shaped member <b>33</b> are rotatably supported with respect to the sleeve <b>23</b> and the cap <b>24</b> through the lubricant <b>41</b>. In the present embodiment, the bearing mechanism <b>4</b> preferably includes: the sleeve <b>23</b> and the cap <b>24</b> which belong to the stationary unit <b>2</b>; the shaft <b>31</b>, the hub <b>32</b> and the ring-shaped member <b>33</b> which belong to the rotary unit <b>3</b>; and the lubricant <b>41</b> existing therebetween. For example, polyol ester-based oil or diester-based oil is used as the lubricant <b>41</b>.

A radial dynamic pressure groove array is provided on at least one of the inner circumferential surface of the sleeve <b>23</b> and the outer circumferential surface of the shaft <b>31</b>. The radial dynamic pressure groove array is formed into, e.g., a herringbone shape. Upon driving the brushless motor <b>11</b>, the radial dynamic pressure groove array induces a radial dynamic pressure in the lubricant <b>41</b> existing between the sleeve <b>23</b> and the shaft <b>31</b>. The shaft <b>31</b> is radially supported with respect to the sleeve <b>23</b> by the dynamic pressure thus induced.

A thrust dynamic pressure groove array is provided on at least one of the upper surface of the sleeve <b>23</b> and the lower surface of the top plate portion <b>61</b>. The thrust dynamic pressure groove array is formed into, e.g., a herringbone shape or a spiral shape. Upon driving the brushless motor <b>11</b>, the thrust dynamic pressure groove array induces an axial dynamic pressure in the lubricant <b>41</b> existing between the sleeve <b>23</b> and the hub <b>32</b>. The hub <b>32</b> is axially supported with respect to the sleeve <b>23</b> by the axial dynamic pressure thus induced.

The magnet <b>34</b> is fixed to the lower surface of the top plate portion <b>61</b> of the hub <b>32</b> by an adhesive agent. The magnet <b>34</b> of the present embodiment is formed into an annular shape and is arranged in a substantially coaxial relationship with the center axis <b>9</b>. The magnet <b>34</b> is positioned above the armature <b>22</b>. The lower surface of the magnet <b>34</b> is axially opposed to the coil patterns <b>223</b> arranged on the upper surface of the armature <b>22</b>. The lower surface of the magnet <b>34</b> is alternately magnetized with N-poles and S-poles along the circumferential direction.

The rotor yoke <b>35</b> is a magnetic body rotating together with the hub <b>32</b> and the magnet <b>34</b>. The rotor yoke <b>35</b> of the present embodiment preferably includes a cylinder portion <b>71</b> and a flange portion <b>72</b>. The cylinder portion <b>71</b> is arranged radially inward of the armature <b>22</b> to axially extend in a substantially cylindrical shape. The cylinder portion <b>71</b> is fixed to the hub protrusion portion <b>62</b>. The flange portion <b>72</b> extends radially outward from the lower end of the cylinder portion <b>71</b>. The flange portion <b>72</b> is positioned below the armature <b>22</b>, above the bottom plate portion <b>51</b> of the base member <b>21</b> and radially inward of the armature holding portion <b>54</b> of the base member <b>21</b>. The flange portion <b>72</b> extends in a substantially annular shape. The upper surface of the flange portion <b>72</b> is axially opposed to the lower surface of the armature <b>22</b>.

In the brushless motor <b>11</b> described above, magnetic fluxes axially penetrating the armature <b>22</b> are generated if a drive current is supplied to the coil patterns <b>223</b> via the power feeding portion <b>224</b>. Also formed is a magnetic circuit extending through the armature <b>22</b>, the magnet <b>34</b>, the hub <b>32</b> and the rotor yoke <b>35</b>. Circumferential torque is generated under the action of the magnetic fluxes. As a result, the rotary unit <b>3</b> is rotated about the center axis <b>9</b> with respect to the stationary unit <b>2</b>. The magnetic disk <b>12</b> supported on the hub <b>32</b> is rotated about the center axis <b>9</b> together with the rotary unit <b>3</b>.

Next, description will be made on the relationship between the hub <b>32</b> and the magnet <b>34</b>.

<figref idref="DRAWINGS">FIGS. 6 and 7</figref> are partial vertical section views of the brushless motor <b>11</b>. <figref idref="DRAWINGS">FIG. 8</figref> is an exploded perspective view of the hub <b>32</b> and the magnet <b>34</b>. <figref idref="DRAWINGS">FIG. 9</figref> is a perspective view of the hub <b>32</b> and the magnet <b>34</b>. <figref idref="DRAWINGS">FIGS. 6 and 7</figref> show vertical cross sections taken in different circumferential positions.

As shown in <figref idref="DRAWINGS">FIGS. 6</figref>, <b>7</b>, <b>8</b> and <b>9</b>, the lower surface of the hub <b>32</b> preferably includes an inner lower surface <b>81</b>, an outer lower surface <b>82</b> and a recess portion <b>83</b>. The inner lower surface <b>81</b> is arranged radially inward of the magnet <b>34</b> and radially outward of the hub protrusion portion <b>62</b> so as to extend in the direction orthogonal to the center axis <b>9</b>. The outer lower surface <b>82</b> is arranged radially outward of the magnet <b>34</b> so as to extend in the direction orthogonal to the center axis <b>9</b>. The recess portion <b>83</b> is positioned between the inner lower surface <b>81</b> and the outer lower surface <b>82</b> and is depressed upward. In the present embodiment, the inner lower surface <b>81</b>, the outer lower surface <b>82</b> and the recess portion <b>83</b> have a substantially annular shape when seen in a bottom view. The inner lower surface <b>81</b>, the outer lower surface <b>82</b> and the recess portion <b>83</b> are arranged in a substantially coaxial relationship with the center axis <b>9</b>.

At least a portion of the magnet <b>34</b> is accommodated within the recess portion <b>83</b>. Thus, the upper surface of the magnet <b>34</b> is positioned above the inner lower surface <b>81</b> and the outer lower surface <b>82</b>. In other words, the axial position of the magnet <b>34</b> partially overlaps with the axial position of the top plate portion <b>61</b>. Therefore, the top plate portion <b>61</b> and the magnet <b>34</b> are restrained from becoming larger in the overall axial dimension. As a result, the brushless motor <b>11</b> is restrained from growing larger in the axial dimension.

In the present embodiment, as shown in <figref idref="DRAWINGS">FIGS. 6 and 7</figref>, the recess portion <b>83</b> is positioned radially inward of the disk support portion <b>63</b>. In other words, the disk support portion <b>63</b> does not axially overlap with the recess portion <b>83</b>. The central lower surface <b>833</b> and the upper surface of the magnet <b>34</b> are positioned above a second support surface <b>632</b>. Therefore, the top plate portion <b>61</b>, the disk support portion <b>63</b> and the magnet <b>34</b> are restrained from becoming larger in the overall axial dimension.

As shown in <figref idref="DRAWINGS">FIGS. 6</figref>, <b>7</b> and <b>8</b>, the recess portion <b>83</b> includes an inner side surface <b>831</b>, an outer side surface <b>832</b> and a central lower surface <b>833</b>. The central lower surface <b>833</b> is arranged above the magnet <b>34</b> to extend in a disc-like shape. The upper surface of the magnet <b>34</b> is axially opposed to the central lower surface <b>833</b>. The inner side surface <b>831</b> is positioned radially inward of the magnet <b>34</b>. The inner side surface <b>831</b> extends downward from the inner circumferential portion of the central lower surface <b>833</b> in a substantially cylindrical shape. The inner side surface <b>831</b> is joined to the outer circumferential portion of the inner lower surface <b>81</b>. The outer side surface <b>832</b> is positioned radially outward of the magnet <b>34</b>. The outer side surface <b>832</b> has a substantially cylindrical shape and extends downward from the outer circumferential portion of the central lower surface <b>833</b>. The outer side surface <b>832</b> is joined to the inner circumferential portion of the outer lower surface <b>82</b>.

A magnetic attraction force is generated between the hub <b>32</b> and the magnet <b>34</b>. As shown in <figref idref="DRAWINGS">FIGS. 6 and 7</figref>, an adhesive agent <b>84</b> exists between the magnet <b>34</b> and the recess portion <b>83</b>. In the present embodiment, the magnet <b>34</b> is fixed to the hub <b>32</b> by the magnetic attraction force generated between the hub <b>32</b> and the magnet <b>34</b> and by the bonding force of the adhesive agent <b>84</b>. Alternatively, the magnet <b>34</b> may be fixed to the hub <b>32</b> only by the magnetic attraction force without having to use the adhesive agent <b>84</b>.

As microscopically shown in the right lower region in <figref idref="DRAWINGS">FIG. 6</figref>, a radial gap <b>92</b> extending over the entire circumference exists between the radial outer end surface of the magnet <b>34</b> and the outer side surface <b>832</b>. In other words, the radial outer end surface of the magnet <b>34</b> does not make contact with the outer side surface <b>832</b>. The radial outer end surface of the magnet <b>34</b> is radially opposed to the outer side surface <b>832</b> across the radial gap <b>92</b>. For this reason, as compared with a case where the magnet makes contact with the outer side surface, magnetic fluxes are difficult to flow from the magnet <b>34</b> toward the outer side surface <b>832</b>. As a result, the magnetic fluxes can efficiently flow from the magnet <b>34</b> toward the armature <b>22</b>.

As microscopically shown in the right lower region in <figref idref="DRAWINGS">FIG. 6</figref>, the lower end of the outer side surface <b>832</b> and the outer lower surface <b>82</b> are positioned above the lower surface of the magnet <b>34</b>. For this reason, as compared with a case where the axial position of the lower end of the outer side surface or the outer lower surface is equal to or lower than the axial position of the lower surface of the magnet, magnetic fluxes are difficult to flow from the magnet <b>34</b> toward the outer lower surface <b>82</b>. In other words, as microscopically shown in the right lower region in <figref idref="DRAWINGS">FIG. 6</figref>, the amount of magnetic fluxes F<b>1</b> indicated by a broken line arrow is decreased. As a result, the magnetic fluxes can efficiently flow from the magnet <b>34</b> toward the armature <b>22</b>. In other words, as microscopically shown in the right lower region in <figref idref="DRAWINGS">FIG. 6</figref>, the amount of magnetic fluxes F<b>2</b> indicated by solid line arrows is increased.

In the present embodiment, as microscopically shown in the left upper region in <figref idref="DRAWINGS">FIG. 6</figref>, the axial position of the inner lower surface <b>81</b> and the axial position of the lower surface of the magnet <b>34</b> are set substantially equal to each other. When the magnet <b>34</b> is attached to the hub <b>32</b>, the magnet <b>34</b> is axially located in position on the basis of the inner lower surface <b>81</b>. Accordingly, the axial distance between the lower surface of the magnet <b>34</b> and the upper surface of the armature <b>22</b> is accurately decided.

As microscopically shown in the left upper region in <figref idref="DRAWINGS">FIG. 6</figref>, the radial inner end surface of the magnet <b>34</b> is radially opposed to the inner side surface <b>831</b> across a gap <b>91</b> in some circumferential positions. In addition, as microscopically shown in the left upper region in <figref idref="DRAWINGS">FIG. 7</figref>, the radial inner end surface of the magnet <b>34</b> makes contact with the inner side surface <b>831</b> in other circumferential positions. In other words, the radial inner end surface of the magnet <b>34</b> makes contact with a portion of the inner side surface <b>831</b> in the circumferential direction. Thus, the magnet <b>34</b> is radially located in position.

In the present embodiment, the radial distance between the radial outer end surface of the magnet <b>34</b> and the outer side surface <b>832</b> is larger than the radial distance between the radial inner end surface of the magnet <b>34</b> and the inner side surface <b>831</b>. The axial position of the outer lower surface <b>82</b> is higher than the axial position of the inner lower surface <b>81</b>. Accordingly, the flow of the magnetic fluxes from the lower surface of the magnet <b>34</b> toward the hub <b>32</b> is more heavily restrained in the outer circumferential portion of the magnet <b>34</b> than in the inner circumferential portion of the magnet <b>34</b>.

The adhesive agent <b>84</b> exists between the upper surface of the magnet <b>34</b> and the central lower surface <b>833</b>. As microscopically shown in the left upper region in <figref idref="DRAWINGS">FIG. 6</figref>, a part of the adhesive agent <b>84</b> exists in the gap <b>91</b> between the radial inner end surface of the magnet <b>34</b> and the inner side surface <b>831</b>. As microscopically shown in the right lower region in <figref idref="DRAWINGS">FIG. 6</figref>, another part of the adhesive agent <b>84</b> exists in the gap <b>92</b> between the radial outer end surface of the magnet <b>34</b> and the outer side surface <b>832</b>. This increases the fixing strength of the magnet <b>34</b> with respect to the hub <b>32</b>.

The adhesive agent <b>84</b> may not necessarily exist in both the gap <b>91</b> and the gap <b>92</b>. In other words, the adhesive agent <b>84</b> may exist in only one of the gaps <b>91</b> and <b>92</b>. If the lower end portion of the adhesive agent <b>84</b> is extruded more downward than the lower surface of the magnet <b>34</b>, there is likelihood that the adhesive agent <b>84</b> makes contact with the upper surface of the armature <b>22</b>. For this reason, it is preferred that the lower end portion of the adhesive agent <b>84</b> be positioned above the lower surface of the magnet <b>34</b>.

In the present embodiment, as microscopically shown in the left upper region in <figref idref="DRAWINGS">FIG. 6</figref> and in left upper region in <figref idref="DRAWINGS">FIG. 7</figref>, the inner side surface <b>831</b> preferably includes an inner joining surface <b>841</b>. The inner joining surface <b>841</b> is positioned in the lower end portion of the inner side surface <b>831</b>. The lower end portion of the inner joining surface <b>841</b> is joined to the outer circumferential portion of the inner lower surface <b>81</b>. The outer diameter of the inner joining surface <b>841</b> becomes smaller downward. For this reason, the radial distance between the inner joining surface <b>841</b> and the radial inner end surface of the magnet <b>34</b> grows larger downward.

In the present embodiment, as microscopically shown in the right lower region in <figref idref="DRAWINGS">FIG. 6</figref>, the outer side surface <b>832</b> preferably includes an outer joining surface <b>842</b>. The outer joining surface <b>842</b> is positioned in the lower end portion of the outer side surface <b>832</b>. The lower end portion of the outer joining surface <b>842</b> is joined to the inner circumferential portion of the outer lower surface <b>82</b>. The inner diameter of the outer joining surface <b>842</b> becomes larger downward. For this reason, the radial distance between the outer joining surface <b>842</b> and the radial outer end surface of the magnet <b>34</b> grows larger downward.

When the magnet <b>34</b> is attached to the recess portion <b>83</b> of the hub <b>32</b>, the magnet <b>34</b> is inserted into the recess portion <b>83</b> along the inner joining surface <b>841</b> and the outer joining surface <b>842</b>. This makes it easy to insert the magnet <b>34</b> into the recess portion <b>83</b>. Even if the uncured adhesive agent <b>84</b> is spread around the lower end portions of the gaps <b>91</b> and <b>92</b>, upward surface tensions act on the adhesive agent <b>84</b> in the vicinity of the inner joining surface <b>841</b> and in the vicinity of the outer joining surface <b>842</b>. This restrains the adhesive agent <b>84</b> from protruding more downward than the lower surface of the magnet <b>34</b>.

As in the present embodiment, each of the inner joining surface <b>841</b> and the outer joining surface <b>842</b> may be a slant surface having a rectilinear shape in a vertical cross section. Alternatively, each of the inner joining surface <b>841</b> and the outer joining surface <b>842</b> may be a curved surface having a round shape in a vertical cross section. One or both of the inner joining surface <b>841</b> and the outer joining surface <b>842</b> may be omitted.

In the present embodiment, as microscopically shown in the left upper region in <figref idref="DRAWINGS">FIG. 6</figref> and in left upper region in <figref idref="DRAWINGS">FIG. 7</figref>, the radial inner end surface of the magnet <b>34</b> preferably includes an inner chamfered portion <b>341</b>. The inner chamfered portion <b>341</b> is positioned in the upper end portion of the radial inner end surface of the magnet <b>34</b>. The upper end portion of the inner chamfered portion <b>341</b> is joined to the inner circumferential portion of the upper surface of the magnet <b>34</b>. The radius of curvature of the inner chamfered portion <b>341</b> becomes larger upward. For this reason, the radial distance between the inner side surface <b>831</b> and the inner chamfered portion <b>341</b> grows larger upward.

In the present embodiment, as microscopically shown in the right lower region in <figref idref="DRAWINGS">FIG. 6</figref>, the radial outer end surface of the magnet <b>34</b> preferably includes an outer chamfered portion <b>342</b>. The outer chamfered portion <b>342</b> is positioned in the upper end portion of the radial outer end surface of the magnet <b>34</b>. The upper end portion of the outer chamfered portion <b>342</b> is joined to the outer circumferential portion of the upper surface of the magnet <b>34</b>. The radial distance between the outer side surface <b>832</b> and the outer chamfered portion <b>342</b> becomes larger upward.

When the magnet <b>34</b> is attached to the recess portion <b>83</b> of the hub <b>32</b>, the inner chamfered portion <b>341</b> or the outer chamfered portion <b>342</b> comes into contact with the hub <b>32</b>, whereby the magnet <b>34</b> is radially located in position. This makes it easy to insert the magnet <b>34</b> into the recess portion <b>83</b>. The adhesive agent <b>84</b> is accumulated in the space between the inner chamfered portion <b>341</b> and the recess portion <b>83</b> and in the space between the outer chamfered portion <b>342</b> and the recess portion <b>83</b>. Thus, the magnet <b>34</b> is strongly bonded to the recess portion <b>83</b>.

As in the present embodiment, each of the inner chamfered portion <b>341</b> and the outer chamfered portion <b>342</b> may be a curved surface having a round shape in a vertical cross section. Alternatively, each of the inner chamfered portion <b>341</b> and the outer chamfered portion <b>342</b> may be a slant surface having a rectilinear shape in a vertical cross section. One or both of the inner chamfered portion <b>341</b> and the outer chamfered portion <b>342</b> may be omitted.

In the present embodiment, as shown in <figref idref="DRAWINGS">FIGS. 6</figref>, <b>7</b>, <b>8</b> and <b>9</b>, the radial length of the inner lower surface <b>81</b> is shorter than the radial length of the outer lower surface <b>82</b>. Thus, the radial position of the magnet <b>34</b> comes closer to the center axis <b>9</b>. If the radial position of the magnet <b>34</b> comes closer to the center axis <b>9</b>, it is possible to reduce the overall radial dimension of the brushless motor <b>11</b>.

In the present embodiment, as shown in <figref idref="DRAWINGS">FIGS. 6</figref>, <b>7</b>, <b>8</b> and <b>9</b>, the hub protrusion portion <b>62</b> is positioned radially inward of the inner lower surface <b>81</b>. In other words, the inner lower surface <b>81</b> exists between the magnet <b>34</b> and the hub protrusion portion <b>62</b>. Therefore, as compared with a case where the inner lower surface <b>81</b> is absent, the magnet <b>34</b> and the hub protrusion portion <b>62</b> are radially spaced apart from each other. This reduces the amount of magnetic fluxes flowing from the magnet <b>34</b> toward the hub protrusion portion <b>62</b>. As a result, the magnetic fluxes can efficiently flow from the magnet <b>34</b> toward the armature <b>22</b>.

In the present embodiment, as shown in <figref idref="DRAWINGS">FIGS. 6 and 7</figref>, the upper end portion of the cylinder portion <b>71</b> of the rotor yoke <b>35</b> makes contact with the inner lower surface <b>81</b>. Thus, the rotor yoke <b>35</b> is axially located in position. The outer circumferential surface of the cylinder portion <b>71</b> is positioned radially inward of the radial inner edge portion of the recess portion <b>83</b>. This reduces the amount of magnetic fluxes flowing from the magnet <b>34</b> toward the cylinder portion <b>71</b>. As a result, the magnetic fluxes can efficiently flow from the magnet <b>34</b> toward the armature <b>22</b>. If the cylinder portion <b>71</b> is positioned radially inward of the recess portion <b>83</b>, the armature <b>22</b> is also arranged radially inward in proportion thereto. This makes it possible to further reduce the overall radial dimension of the brushless motor <b>11</b>.

While illustrative embodiments of the present invention have been described above, the present invention is not limited to the aforementioned embodiments.

<figref idref="DRAWINGS">FIG. 10</figref> is a partial vertical section view of a brushless motor <b>11</b>B according to one modified example. In this modified example, as microscopically shown in the left upper region and the right lower region in <figref idref="DRAWINGS">FIG. 10</figref>, the inner lower surface <b>81</b>B and the outer lower surface <b>82</b>B are positioned above the lower surface of the magnet <b>34</b>B. This makes it possible to reduce the amount of magnetic fluxes flowing from the magnet <b>34</b>B toward the inner lower surface <b>81</b>B and the amount of magnetic fluxes flowing from the magnet <b>34</b>B toward the outer lower surface <b>82</b>B. Accordingly, the magnetic fluxes can efficiently flow from the magnet <b>34</b>B toward the armature <b>22</b>B.

In other words, if at least one of the inner lower surface and the outer lower surface is positioned above the lower surface of the magnet, the amount of magnetic fluxes flowing from the magnet toward the upwardly positioned surface becomes lower than when the axial position of the upwardly positioned surface is equal to or lower than the axial position of the lower surface of the magnet. Accordingly, the amount of magnetic fluxes flowing from the magnet toward the armature is increased.

<figref idref="DRAWINGS">FIG. 11</figref> is a partial vertical section view of a brushless motor <b>11</b>C according to another modified example. In this modified example, as microscopically shown in the left upper region in <figref idref="DRAWINGS">FIG. 11</figref>, a radial gap <b>91</b>C exists between the radial inner end surface of the magnet <b>34</b>C and the inner side surface <b>831</b>C over the entire circumference. In other words, the radial inner end surface of the magnet <b>34</b>C does not make contact with the inner side surface <b>831</b>C. The radial inner end surface of the magnet <b>34</b>C is radially opposed to the inner side surface <b>831</b>C across a gap <b>91</b>C. For this reason, as compared with the structure of the second embodiment, the amount of magnetic fluxes flowing from the magnet <b>34</b>C toward the inner side surface <b>831</b>C becomes smaller.

In this modified example, as microscopically shown in the right lower region in <figref idref="DRAWINGS">FIG. 11</figref>, at least a portion of the radial outer end surface of the magnet <b>34</b>C makes contact with the outer side surface <b>832</b>C. Thus, the magnet <b>34</b>C is radially located in position.

In other words, if a radial gap exists in at least one of a space between the radial inner end surface of the magnet and the inner side surface and a space between the radial outer end surface of the magnet and the outer side surface, the amount of magnetic fluxes flowing from the magnet toward the hub becomes smaller than when the radial gap is absent. Accordingly, the amount of magnetic fluxes flowing from the magnet toward the armature is increased.

<figref idref="DRAWINGS">FIG. 12</figref> is a perspective view of a hub <b>32</b>D and a magnet <b>34</b>D according to a further modified example. In the modified example shown in <figref idref="DRAWINGS">FIG. 12</figref>, the magnet <b>34</b>D preferably includes a plurality of segment magnets <b>343</b>D arranged along the circumferential direction. The lower surfaces of the respective segment magnets <b>343</b>D are magnetic pole surfaces axially opposed to the upper surface of the armature. The segment magnets <b>343</b>D are arranged along the circumferential direction such that magnetic pole surfaces having an N-pole and magnetic pole surfaces having an S-pole are alternately arranged side by side. This helps simplify the shape of the respective segment magnets <b>343</b>D. Accordingly, the respective segment magnets <b>343</b>D can be formed with ease. However, if a single annular magnet <b>34</b> is used as in the second embodiment, it becomes easier to install and locate the magnet <b>34</b>.

The brushless motor according to the preferred embodiments may be a so-called fixed-shaft-type motor in which a shaft belongs to a stationary unit and a sleeve belongs to a rotary unit. The preferred embodiments of the present invention can be used in, e.g., a brushless motor and a disk drive apparatus. The brushless motor and the disk drive apparatus according to the preferred embodiments may be used to rotate a disk other than the magnetic disk, e.g., an optical disk.

The specific shapes of the respective members may differ from those shown in the respective figures of the subject application.

Features of the above-described preferred embodiments and the modifications thereof may be combined appropriately as long as no conflict arises.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.